1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a secondary battery having an electrolyte injection hole having an improved sealing structure to improve the safety of the secondary battery and to prevent the electrolyte from leaking.
2. Description of the Related Art
As portable wireless appliances including video cameras, portable telephones, and portable computers tend to have reduced weight while incorporating more functions, much research has been conducted on secondary batteries which are used as the driving source of the appliances. For example, secondary batteries include nickel cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, and lithium secondary batteries. The lithium secondary batteries are widely used in the cutting-edge electronic appliance fields, because they can be recharged, they can be manufactured in a compact size with large capacity, and they have high operating voltage and high energy density per unit weight.
Such a secondary battery is formed by placing a generator element (i.e., an electrode assembly) including positive and negative electrode plates and a separator into a can made of metal, injecting an electrolyte into the can, and sealing the can. After the can is sealed, an electrode terminal is positioned on top of the secondary battery while being insulated from the can. The electrode terminal acts an electrode of the battery and the can itself acts as the other electrode thereof.
After being sealed, the secondary battery is connected to battery safety devices including a secondary protective device (e.g., a PTC) and a protective circuit module and is placed into a battery pack. The battery safety devices are connected to the positive and negative electrodes, respectively, to interrupt currents when the temperature or voltage of the battery rises due to overcharging/over-discharging and prevent danger, such as fracture of the battery.
In a secondary battery having an electrolyte injection hole formed on the cap plate of a bare cell, the can is a metallic container having a cuboid shape with an open top and is preferably made of aluminum or aluminum alloy which is light and conductive and which has resistance to corrosion. The can acts as a container for an electrode assembly including a positive electrode, a separator, and a negative electrode and for an electrolyte. The electrode assembly is inserted into the can via the open top, i.e., the top opening, which is then sealed by a cap assembly.
The cap assembly is provided with a planar cap plate having a size and shape corresponding to those of the top opening of the can. The cap plate is preferably made of the same material as the can (i.e., aluminum or aluminum alloy) for improved weldability to the can. The cap plate has a terminal through-hole formed at the center thereof so that an electrode terminal can pass through. A tubular gasket is positioned on the exterior of the negative terminal, which passes through the center of the cap plate, for electrical insulation between the negative terminal and the cap plate. An insulation plate is positioned on the bottom surface of the cap plate near the terminal through-hole of the cap plate. A terminal plate is positioned on the bottom surface of the insulation plate.
The electrode assembly is formed by winding the positive and negative electrodes with the separator interposed between them. The positive electrode is electrically connected to the cap plate via a positive electrode tab and the negative electrode is electrically connected to the negative terminal of the cap plate via a negative electrode tab. Therefore, the can is electrically insulated from the negative terminal and acts as a positive terminal. After the cap assembly is welded to the top of the can, an electrolyte is injected via the electrode injection hole of the cap plate. The electrode injection hole is sealed by a plug made of an aluminum ball pressed therein. In addition, a liquid-state resin or resin droplets can be applied on top of the plug and cured by light or heat to prevent the electrolyte from leaking in a two-fold manner.
A lead plate is formed on top of the electrolyte injection hole and is coupled to a separate protective circuit module. The lead plate has a bottom portion having at least a predetermined area for surface-to-surface coupling to the cap plate of the bare cell and an extension portion extending vertically from the bottom portion for coupling to the electrical terminal of the protective circuit module. The extension portion is connected to the electrode tab of the overlying protective circuit module.
However, suchl secondary batteries have a problem in that, since the electrolyte injection hole is sealed by pressing a plug made of an aluminum ball therein, a fine gap tends to exist between the electrolyte injection hole and the plug and the electrolyte can leak through the gap. Particularly, a larger amount of electrolyte is injected into the can, as secondary batteries tend to have larger capacity, and the electrolyte can leak to the top of the electrolyte injection hole due to the capillary phenomenon between the electrolyte injection hole and the pressed ball. Consequently, the welding between the electrolyte injection hole and the ball becomes unstable due to the electrolyte and a pin hole is created at the welded portion.
Furthermore, the thin cap plate deforms as the aluminum ball is pressed into the electrolyte injection hole and so does the electrolyte injection hole. As a result, a fine gap is formed between the electrolyte injection hole and the aluminum ball even when the aluminum ball is pressed into the electrolyte injection hole and the electrolyte can leak.